Having recently built my first aquaponics system, I wanted to be able to easily test the water conditions. This led to build a handheld pH probe.
The probe is powered with an Arduino Pro Mini from SparkFun and a pH probe and stamp (a tiny microcontroller) from Atlas Scientific.
There is a user interface to calibrate the pH probe using a waterproof temperature sensor. Two buttons provide the interface while an LCD prints info out for the user to read.
The the entire build in the video below.